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   难度：Medium
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   <h1 class="question_title">
    357. Count Numbers with Unique Digits
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   <p>
    Given a
    <b>
     non-negative
    </b>
    integer n, count all numbers with unique digits, x, where 0 &le; x &lt; 10
    <sup>
     n
    </sup>
    .
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     <strong>
      Example:
     </strong>
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    <pre>
<strong>Input: </strong><span id="example-input-1-1">2</span>
<strong>Output: </strong><span id="example-output-1">91 
<strong>Explanation: </strong></span>The answer should be the total numbers in the range of 0 &le; x &lt; 100, 
&nbsp;            excluding <code>11,22,33,44,55,66,77,88,99</code>
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   <h1 class="question_title">
    357. 计算各个位数不同的数字个数
   </h1>
   <p>
    给定一个
    <strong>
     非负
    </strong>
    整数 n，计算各位数字都不同的数字 x 的个数，其中 0 &le; x &lt; 10
    <sup>
     n&nbsp;
    </sup>
    。
   </p>
   <p>
    <strong>
     示例:
    </strong>
   </p>
   <pre><strong>输入: </strong>2
<strong>输出: </strong>91 
<strong>解释: </strong>答案应为除去 <code>11,22,33,44,55,66,77,88,99 </code>外，在 [0,100) 区间内的所有数字。
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